The present invention relates to a pneumatic tire, more particularly to a heavy duty radial tire having an improved bead and lower sidewall structure capable of improving the bead durability.
1. Field of the Invention
In radial tires for heavy duty vehicles such as trucks and buses, the bead portion and sidewall lower portion are subjected to a large bending deformation, and accordingly, the outer surface thereof is liable to crack, which sometimes cause further damages such as carcass ply loose. Therefore, if the bead or sidewall surface is cracked, it is impossible to recycle such tires by retreading.
2. Description of the Related Art
In order to improve bead durability by increasing the rigidity of bead portions, hitherto, as shown in FIG. 10(A), a bead apex rubber (b) which is inserted between the carcass main portion (a1) and turnup portion (a2) is increased in the volume and height.
In such a conventional structure, however, under severe service conditions, the increased rubber volume increases heat generation. Further, when the sidewall portions are greatly bent axially outwardly, the carcass ply turnup portion (a2) is subjected to a large compression strain, and this portion is liable to crease. Furthermore, as the radially outer end of the turnup portion approaches to the tire outer surface, and a relatively large stress concentrates on the end, carcass ply edge separation, cord loose and cracks are liable to occur. Thus, the bead durability is lost quickly.
It is therefore, an object of the present invention to provide a heavy duty radial tire, in which internal temperature rise, crease of carcass turnup portion, carcass ply edge separation, cord loose and sidewall cracks are effectively controlled to improve the bead durability.
According to one aspect of the present invention, a heavy duty radial tire comprises
a tread portion,
a pair of sidewall portions,
a pair of bead portions with a bead core therein,
a carcass comprising a ply of cords arranged radially at an angle of from 70 to 90 degrees with respect to the tire equator extending between the bead portions through the tread portion and sidewall portions and turned up around the bead cores form the axially inside to outside of the tire to form a pair of turnup portions and a main portion therebetween,
a bead apex disposed between the main portion and each turnup portion of the carcass ply and extending radially outwardly from the bead core, and
a chafer disposed in each bead portion to define a rim contacting surface,
each turnup portion extending radially outwardly beyond the radially outer end of the bead apex so that the turnup portion has a parallel part which is placed adjacent to and substantially in parallel with the main portion,
the chafer extending radially outwardly along the axially outside of the turnup portion beyond the radially outer end of the bead apex.
Therefore, the bead apex height decreases and the rubber volume between the carcass turnup portion and main portion is decreased. Contrary, the rubber volume on the axially outside of the carcass turnup portion is increased by the chafer. Accordingly, the main heat source shifts from the bead apex to the chafer, and the generated heat easily radiates. Further, by the existence of the parallel part, the turnup portion is prevented from being subjected to compressive strain even if the tire is greatly deformed. Therefore, internal temperature rise, carcass turnup crease, carcass ply edge separation, cord loose and sidewall cracks can be prevented, and the bead durability is effectively improved.
An embodiment of the present invention will now be described in detail in conjunction with the accompanying drawings.